1. Field of the Invention
The present invention relates to a vibration characteristic measuring apparatus that measures vibration characteristics of a rotational shaft of a rotating machine like a multi-stage centrifugal compressor and a vibration characteristic measuring method.
2. Description of the Related Art
A multi-stage centrifugal compressor, which is installed in a petroleum field, a natural gas plant, and a petrochemical plant, etc., and which compresses a compressible fluid, such as hydrocarbon or carbon dioxide gases, rotates centrifugal impellers of multi-stages along with a rotational shaft, and compresses the compressible fluid by centrifugal force.
Such a multi-stage centrifugal compressor is provided with a sealing mechanism that has an annular sealing member around the axial line of the rotational shaft so as to suppress the leak of the pressurized fluid from each stage of the centrifugal impeller.
According to the sealing mechanism of the multi-stage centrifugal compressor, the instability factor due to the leak and flow of the pressurized fluid occurs, which affects the stability of the rotational shaft and the centrifugal impeller. That is, when the instability factor exceeds the stability factor, self-induced vibration so-called seal whirl occurs.
That is, it is important to design the rotating machine so as to have larger stability sufficiently than instability generated at the sealing mechanism. In order to do so, it is necessary to measure the vibration characteristics of a rotating body including the rotational shaft to find out the characteristics thereof, and to evaluate the stability based on the measured vibration characteristics.
For example, JP H05-5057 B discloses a technique of applying a vibration to a bearing, and analyzing the vibration generated by a rotational shaft due to the former vibration, thereby measuring the vibration characteristics of a rotating machine in operation. Also, Pettinato, et al., “Shop Acceptance Testing of Compressor Rotordynamic Stability and Theoretical Correlation”, 39th Turbo Machinery Symposium, Texas, 2011, p.p. 31-42 discloses a technique of attaching an active radial magnetic bearing to an end of a rotational shaft of a compressor, supplying an excitation current to the magnetic bearing to apply vibration to the rotational shaft, thereby measuring the vibration characteristics of the rotational shaft.
The above-explained two techniques are to apply vibration to a rotating rotational shaft, measure the vibration (an amplitude and a phase) of the rotational shaft, thereby measuring the vibration characteristics of the rotational shaft.
When the vibration characteristics of a rotating body like the rotational shaft is measured in this manner, in order to obtain high-quality data with little variability, it is preferable that the amplitude of the applied vibration should be large. By increasing the vibration amplitude of the rotating body excited by the applied vibration, the S/N ratio of a signal output from a sensor measuring the vibration is improved, and thus high-quality data indicating the vibration characteristics can be obtained.
On the other hand, when the vibration amplitude of the rotating body is increased by the applied vibration, if it exceeds an allowable value set as a designed value, for example, the rotating body contacts the sealing mechanism, and the sealing member of the sealing mechanism is worn out. Hence, it is necessary to apply vibration within a range where the vibration amplitude of the rotating body does not exceed the allowable value.
However, the rotating body of the rotating machine is vibrated within a range where the vibration amplitude does not exceed the allowable value in the normal rotation due to an unbalanced weight, etc. Accordingly, the amplitude of the vibration that can be excited by applying vibration to the rotating body is restricted within the range of a margin from the amplitude of the vibration at the non-excited condition to the allowable value. In other words, the vibration excited by the applied vibration at the rotational shaft is restricted and small, and thus the S/N ratio of a signal output from the sensor measuring the vibration becomes poor. Hence, the quality of data (data indicating the vibration characteristics of the rotational shaft) obtained based on such a signal deteriorates.
For example, when the quality of obtained data is poor, vibration analysis can be made by largely increasing the number of data to be obtained and averaging those pieces of data.
However, when the large numbers of data are obtained for averaging, it takes a long time to obtain the large numbers of data and the number of process steps is increased. Also, the operation time of the rotating machine and the time for applying vibration become long in order to obtain the large numbers of data, resulting in the increase of energy consumption.
Therefore, it is an object of the present invention to provide an apparatus and a method for measuring vibration characteristics which can vibrate a rotating body without exceeding an allowable value when measuring vibration characteristics, and which can obtain high-quality data without increasing energy consumption.